Skin, tanning is generally perceived to enhance personal appearance. The skin plays an important role in our social interaction. Since it is our outer covering, skin provides the first visual impression of our individuality. Its appearance often generates judgements on sensual appeal, beauty and even health. A suntan has integrated itself into the appearance aesthetics of skin with the result that suntanned skin is regarded not only as desirable and attractive but also possesses the attribute of health. As a result, many individuals actively seek a tan by exposing themselves to the sun. Such exposure, as is well known from human experience, often results in painful erythema or sun burn. It may also result in premature aging of the skin, certain types of dermatological diseases and even skin cancer.
The art has, therefore, long sought chemical agents which will impart to the skin the characteristic bronze color of a natural tan, but will do so without extensive and potentially dangerous exposure or with minimum exposure to the sun. Such agents must be easily available, inexpensive, non-toxic, stable on storage and capable of imparting an even, long lasting natural-looking coloration to the human skin.
One of the first agents found to be effective in generating a tan coloration to the human skin was dihydroxyacetone. Unfortunately, this material was found to be more reactive with some areas of the skin than others resulting in a speckled appearance, and, in some cases, an unpleasant yellowish color.
Tanning is believed to be the result of melanin formation, a reaction which is triggered by the action of sunlight, principally the UV component of sunlight.
The actual mechanism by which melanin forms in the human body is not a part of this invention. It believed that melanin is a polymer containing several indole segments formed as a result of the cyclization and subsequent polymerization of dihydroxyphenylalanine (DOPA) caused by exposure to the sun.
Not surprisingly, the art has turned to indoles as of possible utility in coloring the skin. As a result, it has been observed that many indoles will cause the topically formed melanin to give a tanned appearance to the skin. European Patent Publication 0,239,896 is representative of one such effort. That patent describes the coloration of skin by the use of such indoles.
Coloration by indoles is believed to occur by photolytic oxidation under the influence of sunlight or some other source of ultraviolet light. The reaction is very slow and it may take very large doses of energy to convert the indoles to a detectable tan. As a result the skin remains unprotected and erythema develops.
On the earth's surface, sunlight spectrum ranges from 290 nm to 1,850 nm in wavelength. Infrared light, which has the lowest energy in the sunlight spectrum, covers from 1,850 nm to ca. 750 nm. Visible light ranges from ca. 750 nm to 400 nm. Sunlight with even higher energy is known as ultraviolet light, and it is further divided into UVA (400 nm-320 nm) and UVB (320 nm-290 nm). It is believed that UVB exposure is principally responsible for the symptoms of sunburn or other skin diseases. However, it is now thought that UVB and UVA combined are more deleterious than either one alone. Therefore, recent efforts in the sunscreen industry are focused on formulating sunscreens capable of blocking both regions of the ultraviolet spectrum.